In general, an adhesive sheet includes a base sheet; an adhesive layer formed on a surface of the base sheet; and a release sheet which is optionally provided on the adhesive layer. In use of such an adhesive sheet, when the adhesive sheet includes a release sheet, the release sheet is removed therefrom, and the adhesive sheet is applied to an adherend by bringing the adhesive layer into contact with the adherend.
However, in the case of use of an adhesive sheet having a somewhat large area (e.g., an adhesive sheet for identification or decoration, an adhesive sheet for masking, or an adhesive sheet for protecting the surface of a metal plate or the like), the adhesive sheet poses a problem in that air bubbles are likely to be formed between the adhesive layer and an adherend, and the thus-formed air bubbles become “blisters”; i.e., the adhesive sheet is difficult to apply smoothly to the adherend.
In order to solve such a problem, attempts have been made to provide an adhesive sheet including an adhesive layer having air passages in a surface portion thereof, so that when the adhesive sheet is applied to an adherend, formation of air bubbles is prevented by releasing air through the flow passages (see, for example, Patent Document 1). Further, there have been proposed an adhesive sheet in which the arrangement (or arrangement pattern) of air passages (grooves) is defined such that a groove is necessarily present in any circular region having a diameter of 500 μm; i.e., the distance between adjacent grooves is small (Patent Document 2); and an easily applicable adhesive sheet in which grooves are provided in a lattice pattern so as to extend obliquely with respect to the direction of application of the adhesive sheet (Patent Document 3).
However, generally, such an adhesive sheet poses a problem in that when grooves (i.e., air passages) have a small width, it is difficult to release air therethrough, whereas when the grooves have a large width, the adhesive sheet exhibits poor appearance due to formation of dents on the surface of the base sheet. Particularly, when the distance between adjacent grooves is increased for achieving sufficient adhesion, since the width of the grooves must be increased, the adhesive sheet exhibits poor appearance due to formation of dents on the surface of the base sheet.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. S63-223081    Patent Document 2: Japanese Kohyo Patent Publication No. 2002-544364    Patent Document 3: WO 03/025078 pamphlet